1. Field of the Invention
The present invention relates generally to amplifiers; and more particularly, to amplifiers for mobile communication systems using radio frequency multiple access signals.
2. Description of the Prior Art
In the prior art, class A and F amplifiers for amplifying a radio frequency signal in a base station of a mobile radio communication system are known in the art. However, with today's high peak to average power signals, neither a class A nor a class F amplifier provides adequate harmonic filtering of the radio frequency signal. Because of this, one disadvantage of the prior art class A and F amplifiers is that the harmonics in the radio frequency signal cause power interference between adjacent channels, resulting in degraded mobile communication service, including more dropped calls and a reduced calling range.
For example, the class A amplifier has appreciable power gain, produces a magnified replica of the input signal voltage or current wave, and is typically used in multimode or multi-signal transmitters where accurate reproduction of both the envelope and the phase of the signal is required. Tuned circuits or low-pass filters are not integral components of class A amplifiers, although they may be included to improve harmonic suppression. An example of the class A amplifier is shown and described on pages 352-355 (see FIG. 12-3) of H. L. Krauss, Solid State Radio Engineering, John Wiley and Sons, New York, 1980.
The nonlinearity in class A amplifiers produces two types of unwanted signals, called harmonics and intermodulation distortion (IMD) products. The IMD products (identified as third, fifth, seventh order products) are prominent near the carrier frequency. They cause distortion in the received signal and/or adjacent channel interference. In a typical radio frequency power amplifier, the unwanted harmonics may be removed by filters, but the unwanted IMD products generally cannot be removed. Current code division multiple access (CDMA) amplifiers have a 15 dB peak to average ratio which greatly increases the IMD and harmonics.
The class A amplifier also dissipates large amounts of power to amplify a signal due to the biasing operation in the center of a typical power amplifier load line. Thus the class A amplifier is not an efficient amplifier. Furthermore, the class A amplifier cannot suppress IMD products due to a lack of harmonic traps.
Furthermore, the class A amplifier under large signal conditions tends to self bias. This self bias moves toward shutting the device off, generating further harmonics and intermodulation products. A large signal condition is defined as an input signal that is strong enough to distort the output signal beyond the IS97 specification.
In comparison, the class F amplifier is characterized by a load network that has resonances at one or more harmonic frequencies as well as the carrier frequency. The class F amplifier has a transistor that usually operates as a current source or a saturating current source, similar in operation to a classical class C power amplifier. A third-harmonic peaking amplifier provides an example of the operation of the class F amplifier. The transistor acts as a current source, producing the same half-sine-wave similar in operation to a class B amplifier. The fundamental-frequency tune circuit bypasses the harmonics, producing a sinusoidal output voltage. Generally, the third-harmonic resonator makes possible a third-harmonic component in the collector voltage, which flattens the collector voltage. An example of a class F amplifier is shown and described on pages 454-458 (See FIG. 14-9) of H. L. Krauss, Solid State Radio Engineering, John Wiley and Sons, New York, 1980.
But the class F amplifier does not amplify a signal linearly. The class F amplifier is essentially biased off, and strongly flattens the signal which introduces large amounts of IMD distortion. Thus the class F amplifier is not linear. Furthermore, the class F amplifier will not amplify low power signals. Thus if a signal is 10 dB below normal, then the class F amplifier will not amplify it. For example, CDMA signals with peak to average power ratios of 15 dB cannot be amplified linearly. The class F amplifier also has a radio frequency choke (RFC) which allows undesirable radio frequency harmonics to travel unfiltered through the amplifier.
In view of the above, there is a real need in the prior art for an amplifier that effectively filters harmonics from the radio frequency signal and also suppresses IMD products.